One liter of paint covers 100 square feet. how many gallons of paint does it take to cover a room whose walls have an area of 800 square meters?

1 gallon is 3.78 L
100 sq feet needs 1 L
800 sq feet needs x

100/800 = 1/x
100x = 800
x = 800/100
x = 8 L

If you use the fact that 4 L = 1 gallon then you need 2 gallons of paint. I you use the more accurate 3.78 L then
1 gallon = 3.78 L
x gallon = 8 L

3.78 x = 8*1
x = 8/3.78
x = 2.12 gallons.
 
I guess you could say that 1 gallon and 1 L would do you.

That they are numbers and related to each other.